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As ultraviolet (UV) radiation has been implicated in recent reports of “environmental stress” in shallow water marine ecosystems, we have studied the effect of UV on two symbiotic associations; the temperate dinoflagellate Symbiodinium californium and its host Anthopleura elegantissima, and the tropical dinoflagellate S. microadriaticum and its host Cassiopeia xamachana. The algae were exposed to UV in the presence of photosynthetically active radiation (PAR) in culture and in hospite, and their responses compared to cells exposed to PAR only. Exposure to UV affected the size of neither S. californium nor S. microadriaticum, but growth rates in culture were significantly reduced under the influence of UV in the former, but not the latter. Synthesis of chlorophylls a and c₂ was depressed by UV in S. microadriaticum, but not in S. californium. UV depressed cell motility in culture in both species. While S …